The present invention relates to a noise reduction device and a semiconductor device having the noise reduction device. More specifically, the present invention relates to a noise reduction device including a plurality of noise reduction circuits and a semiconductor device having the noise reduction device.
In general, a conventional semiconductor device may include a conventional noise reduction circuit for reducing an external turbulent noise, a power source noise, and the like superimposed on an input signal. Such a conventional noise reduction circuit may include a Schmitt circuit and a low pass filter formed of a capacitance element and a resistor element.
Patent Reference 1 has disclosed such a conventional noise reduction circuit. The conventional noise reduction circuit is formed of a first delay circuit for receiving an input signal and outputting a first delayed signal delayed by a first delayed amount; a first logic circuit for receiving the input signal and the first delayed signal and performing a first logic calculation to output a first signal; a second delay circuit for receiving the first signal and outputting a second delayed signal delayed by a second delayed amount; and a second logic circuit for receiving the first signal and the second delayed signal and performing a second logic calculation to output a second signal. In the conventional noise reduction circuit disclosed in Patent Reference, it is configured such that the second delayed amount is greater than the first delayed amount.    Patent Reference: Japanese Patent Publication No. 2009-55470
An example of a conventional semiconductor device 110 having the conventional noise reduction circuit will be shown in FIG. 8. FIG. 8 is a circuit diagram showing the conventional semiconductor device 110 having a conventional noise reduction circuit 127 and an external circuit 115.
As shown in FIG. 8, the external circuit 115 is disposed outside the conventional semiconductor device 110. Further, an output signal of the external circuit 115 is input into the conventional noise reduction circuit 127 through an input terminal 123.
The external circuit 115 has a function of outputting a signal input into the conventional semiconductor device 110 through the input terminal 123. As shown in FIG. 8, the external circuit 115 is a crystal oscillation circuit including an inverter 133, a crystal oscillation element 137, a resistor element R11, a capacitor element C11, and a capacitor element C12. In the external circuit 115, when the capacitor element C11 and the capacitor element C12 are repeatedly charged and discharged, the crystal oscillation element 137 is oscillated to perform an oscillation operation. Then, the inverter 133 amplifies an oscillation signal, and the external circuit 115 outputs the oscillation signal.
Accordingly, the oscillation signal output from the external circuit 115 is input into the input terminal 123 of the conventional semiconductor device 110. When the oscillation signal is input into the input terminal 123, an external turbulent noise due to an external turbulence and the like may be superimposed on the oscillation signal.
As shown in FIG. 8, the conventional noise reduction circuit 127 includes a low pass filter formed of a resistor element R12 and a capacitor element C13 for removing noise superimposed on the oscillation signal, and a Schmitt circuit 143.
In the conventional noise reduction circuit 127, a time constant of the low pass filter 141 and a Schmitt width of the Schmitt circuit 143 are adjusted according to the noise superimposed on the oscillation signal. Accordingly, the conventional noise reduction circuit 127 passes the oscillation signal in an original form to be output to an internal circuit (not shown), while removing the noise superimposed on the oscillation signal.
In the conventional noise reduction circuit 127 shown in FIG. 8, however, it is difficult to remove a noise (a power source noise) superimposed by a power source and accompanied by a fluctuation of the power source. Accordingly, the oscillation signal thus output may be inverted due to the power source noise.
FIG. 9 is a chart showing an example of a relationship among a power source voltage, threshold values of the Schmitt circuit 143, a voltage of a node A, and a voltage of a node B in the conventional noise reduction circuit 127 when the power source noise is generated.
As shown in FIG. 9, when the signal input from the external circuit 115 through the input terminal 123 has a high (H) level, if the power voltage is suddenly increased due to the power source noise, the input of the Schmitt circuit 143 (the node A) does not follow the sudden increase due to a filter characteristic.
On the other hand, the threshold values (the Schmitt threshold value H and the Schmitt threshold value L) of the Schmitt circuit 143 vary following the sudden increase in the power source voltage. Accordingly, the voltage of the note A becomes smaller than the threshold values of the Schmitt circuit 143. As a result, when the Schmitt circuit 143 outputs the output signal having the high (H) level, an unexpected glitch having a low (L) level pulse is generated in the output of the Schmitt circuit 143 (the node B).
When the unexpected glitch is generated and the output signal is inverted, a whole system (the internal circuit) may malfunction.
Further, in general, there have been many cases in which a same noise reduction circuit is used relative to different input signals when an extent of the noise is small as long as the noise is a similar type. For example, there have been many cases in which it is possible to use the conventional noise reduction circuit 127 described above. However, when the noise reduction circuit is used at the input terminal (the input pad), it is difficult to use the same noise reduction circuit due to other restrictions according to the input signal if the extent of the noise is not small.
In view of the problems described above, an object of the present invention is to provide a semiconductor device and a noise reduction device capable of solving the problems of the conventional noise reduction device. In the present invention, it is possible to reduce the noise more effectively through applying various measures depending on a type of an input terminal according to an input signal relative to the noise accompanying the fluctuation in the power source voltage.
Further objects and advantages of the invention will be apparent from the following description of the invention.